The immersion microscope objective of the invention is assigned to the class known as “planapochromat”. A planapochromat is characterized in that the focus positions of the spectral lines e, C′ and F′ are coincident. The “plan” in planapochromat means “planar” and that the image field is flattened.
Water, glycerine and oil can be used as immersion mediums whereby the immersion microscope objective is especially suitable for live cell imaging methods. Here, it is necessary that the indices of refraction of the liquids on both sides of the cover glass approximate each other. The immersion medium “water” is suitable for the examination of living objects especially when the objective is to be immersed directly into the aqueous medium without cover glass. In contrast, for critical fluorescence examinations, a purified glycerine is preferable as the immersion medium because purified glycerine has virtually no inherent fluorescence.
Glycerine and water are preferably utilized in the microscopy of living objects because these objects are in a medium having a similar index of refraction.
Furthermore, it is desirable that the objective also is usable with oil as an immersion medium so that the objective can be used for other usual viewing. Furthermore, a large work distance is wanted because objectives having a large working distance ensure easy accessibility to the specimen.
Various immersion microscope objectives are known from the state of the art. Thus, a microscope objective having three different variations with respective numerical apertures of 1.15 is shown by way of example in U.S. Pat. No. 5,530,590. This objective comprises three lens groups. The second lens group can be displaced along the optical axis relative to the two other lens groups in order to adapt the objective to the thickness of the cover glass and to so compensate the spherical and chromatic aberration which varies with this thickness.
In United States patent publication US 2006/0087745 A1, an immersion microscope objective is likewise described which, however, does not yet satisfy the requirements in many applications with respect to the correction of the longitudinal chromatic aberration or the planapochromatic correction. Furthermore, this objective is not corrected for oil as an immersion medium.